This invention relates to a method for improving the printability characteristics of gloss calendered paper such as label paper or other paper which is coated on one side only. Such papers are normally prepared by applying to one surface of a suitable paper substrate a liquid coating composition. A variety of conventional application techniques using devices such as trailing blade coaters, air knife coaters, roll coaters and such are used. The coating normally includes a pigment, such as clay or titanium dioxide, and a binder which joins the pigment to the surface of the paper. Casein and various starches are well known naturally-occuring binders although numerous other binders, both natural and synthetic, are known to those skilled in the art. The coated paper is then normally dried to a moisture content less than about 80%, and typically about 4 to 6%, to set the coating and render the paper suitable for further processing to develop its printability characteristics.
Coatings are applied to paper in order to improve its appearance and printability characteristics. to maximize these properties, the common practice is to subject the coated paper to further processing to improve such properties as the gloss of the paper and the surface characteristics of the coating so that it becomes more receptive to print.
Several techniques are conventionally used for this purpose, with the degree of improvement which is obtained normally depending upon the particular tenchique chosen. One technique is referred to by those skilled in the art as "gloss-calendering." In this process, the paper is passed through a nip formed between a heated highly polished revolving non-deformable roll (which in some cases may be a chrome plated roll revolving at high speed) and one or more deformable rolls which can be made from a variety of materials. The relatively dry coated surface of the paper is pressed against the surface of the highly polished roll in the nip and takes on, as nearly as possible, the polished surface of the roll. This produces a coated paper of improved gloss and printability characteristics. In the gloss-calendering operation, it is well known that the coating must have a low moisture content, e.g., 4-6% by weight. Otherwise, the coating tends to stick to the rolls during the calendering.
In another technique commonly referred to as "cast coating", the coated surface of the paper is pressed against the highly polished surface of a heated revolving drum while the coating is still quite wet (usually containing about 25 to 40% moisture at the drum surface) and the coating is dried while in contact with the heated drum. The coating takes on the highly polished surface of the drum as it dries and produces a paper having superior gloss and printability characteristics. A disadvantage of this process, however, is its slowness necessitated by the residence time on the drum required to dry the coating. Thus the cast coating process typically processes the coated paper at only about 1/4 to 1/2 the speed of a normal coater operation. Production is usually much slower in a cast coating process than, for example, in a conventionall trailing blade coating operation.
Another method of improving the appearance and printability of coated paper is the well known "supercalendering" operation in which the paper is passed through a stack of high pressure alternating metal and deformable rolls which cause the paper to slide faster on one side than the other as it passes through the nips of the rolls. This improves the gloss and printability characteristics of the paper but by a mechanism different than that of the gloss calender. In the supercalender, where polished rolls are not used, the improved properties result from the creep caused by the slippage of the paper in the nip, whereas in the gloss calender, the coated surface simply takes on the smooth polished surface of the drum it is pressed against. While the supercalender generally produces paper of better print quality than a gloss calender, it has several disadvantages. First, it is a difficult machine to operate and control. Paper has a tendency to stick to the rolls and break, necessitating a time-consuming rethreading of the entire stack of rolls. Moreover, supercalenders are usually not located in line with the coating equipment in most paper-making plants, resulting in process inefficiency and increased product waste. Finally, because of the high pressures required, the paper is significantly reduced in caliper making it less stiff and therefore more difficult to feed to a printing press. This undesirably slows down the operating speed of the press.
Supercalendered paper normally has gloss and printability characteristics superior to gloss calendered paper. However the gloss calender has several advantages over a supercalender which would make it an attractive replacement for the supercalender if both machines produced a product of comparable quality. For example, the gloss calender is a less expensive and complicated machine, and is significantly easier to operate. It usually runs with less paper breaks and can be quickly restarted should the paper break. Since it operates at a lower pressure than the supercalender, it does not reduce the thickness of the paper as much, producing a stiffer sheet which can be fed to the printing presses at a faster rate than supercalendered paper. Gloss calendered papr also normally has a higher backside roughness which reduces the tendency of the paper to stick together, again providing for better feeding of the paper to a printing press. Finally, gloss calenders are typically located directly in line with the coating equipment in many paper-making plants so that paper from the coating equipment can proceed directly and continuously to the gloss calender for maximum process efficiency and minimum product waste. Numerous advantages would be apparent to those skilled in the art if a means could be found to use a gloss calender to produce paper whose quality approximated or equalled that of supercalendered paper.
It is a general object of this invention, therefore, to provide a method whereby paper coated on one side can be processed in a conventional gloss calender to produce a paper product whose printability characteristics approximate or equal those of supercalendered paper.
It is another object of this invention to provide a method for improving the print smoothness of gloss calendered paper to a level which approximates or equals that of supercalendered paper.
These and other objects of the invention will be apparent to those skilled in the art upon a consideration of this entire specification.